In position-measuring systems, position sensors in a measuring device generate electrical signals, which provide information on the position of objects which are moving in relation to one another. The present invention relates to measuring systems that use measuring devices which produce comparatively precise, incremental positional information, as well as relatively rough positional information. These two types of positional data may be of particular importance for controlling the electric drives used in moving the axes of a processing machine, such as of a machine tool or robot. In such applications, the precise, incremental, positional information may be used to precisely determine positions, for example, of a tool of a machine tool.
Such electric drives may be arranged as rotary electromotors, for which rotary transducers may be used to perform angle-of-rotation measurements. Example embodiments of the present invention may also be used in connection with the operation of linear motors.
Rotary transducers may enable angular position measurements to be taken on a rotatable shaft in incremental measuring steps. However, so-called absolute-value encoders, also described as rotary, shaft-angle or incremental encoders, are also conventional. These permit an absolute angular determination to be made within one single shaft rotation. Moreover, if the need arises to determine the number of completed shaft rotations, then so-called multiturn rotary encoders may be used. Such multiturn rotary encoders determine the absolute angular position within one shaft rotation, i.e., between 0° and 360°, using an encoder disk which is connected to the shaft and is scanned by a suitable photoelectric scanning unit. Thus, it is also possible to measure the absolute position of the driven shaft over a plurality of rotations.
The signals of these measuring devices may be used for controlling the processing machines. The term processing machine is not limited to machine tools, but also includes machines for populating electronic components or for machining semiconductor elements. Automation or programmable machines, such as robots, also fall under the processing-machine designation.
In conventional position-measuring systems, in addition to the digital positional data, analog position signals are also transmitted from the measuring device to the machine control, where these signals are then interpolated. Due to the advancing miniaturization of electronics, these interpolation processes are being increasingly carried out in a suitable electronic circuit within the measuring device itself, so that the analog position signals are not transmitted to the machine control. This may reduce the outlay for wiring, which may have a significant effect on the costs of a measuring system.
However, in safety-related machine applications under conventional methods, the digital positional data and the analog position signals are compared in the machine control in order to detect errors. However, since analog position signals may be missing in the machine control, it may no longer be possible for this comparison to be made.
Therefore, in measuring systems, in which no analog position signals arrive in the machine control for the mentioned reasons, besides the current, often absolute positional data, it may not be unusual for so-called static bits to be transmitted via a parallel or serial interface from the measuring device to the machine control. These static bits may be error bits, for example, which, in normal operation, always exhibit a specific level, and only in a (very rare) case of error, point to an error due to a change in level.
This manner of transmitting error information may be disadvantageous, e.g., in the case of safety-related monitoring, because it may not be ruled out that a defect may cause a constant level of an error bit to continually be output, thus that this defect does not permit a change in level even in the case of faults or malfunctions.
German Published Patent Application No. 38 29 815 of the assignee hereof describes a position-measuring device or position transducer, where a test for errors is initiated by an activation signal. However, the performance reliability of the monitoring electronics itself may not be checked. Moreover, the outlay entailed for signal transmission may be comparatively large.